Method of producing carbon nanotubes

ABSTRACT

The present teachings are directed to methods of preparing cylindrical carbon structures, specifically single-walled carbon nanotubes, with a desired chirality. The methods include the steps of providing a catalyst component on a substrate and a carbon component, contacting the catalyst component and the carbon component to produce a cylindrical carbon structure. Then, no longer providing the carbon component and determining the chirality of the cylindrical carbon structure. The catalyst component is then cleaned and the process is repeated until the cylindrical carbon structure fulfills a desired characteristic, such as, length. The chirality of the single-walled carbon nanotube grown, after cleaning of the catalyst component, has the same chirality as the initially produced nanotube.

BACKGROUND

1. Field of the Invention

The present teachings relate to methods of producing carbon nanotubes from initially produced nanotubes so that the subsequently produced nanotubes have the same chirality as the initially produced nanotubes.

2. Discussion of the Related Art

The desire to produce cylindrical carbon structures, specifically carbon nanotubes, and more specifically, single-walled carbon nanotubes (hereinafter “SWNT”), with a specific chirality has been an unfilled desire since it was realized that the chirality of the nanotube influences or controls numerous nanotube properties.

Smalley et al. have described a method of “cloning” SWNT grown by a CVD based method by growing SWNT fibers with open ends, reductively docking nanosized transition metal particles to the open ends of the SWNT fibers and restarting growth of the SWNT on the exposed metal particles. The SWNT growth from the docked nanocatalysts is said to have the same diameter and chirality (n,m) as the base SWNT. See Nanoletters, Vol. 5, No. 6, June 2005, pp. 997-1002.

The total amount of SWNT that could be grown by prior methods of growing SWNT using metal catalysts was limited by the build-up and coating of the metal catalyst with a layer composed of, among other compounds, amorphous carbon and metal carbides. Additionally, the methods of growing the SWNTs did not offer means of controlling the chirality of the SWNT produced.

SWNTs have attracted attention because of their unique chemical and physical properties. A carbon nanotube can be described as a rolled-up graphite sheet in which hexagonal-shaped units of carbon atoms are bound to each other with very strong bonds between the carbon atoms. SWNTs have minimum diameters of about 0.4 nm with lengths ranging as long as several hundred micrometers with extremely small dimensional fluctuations. The electrical conductivity of carbon nanotubes range from a semiconductor to a metal depending upon the chirality of nanotube.

Chirality of a nanotube is denoted by a double index (m,m) where n and m are integers that describe how a single strip of hexagonal “chicken-wire” graphite is cut so it forms a tube that wraps perfectly onto the surface of a cylinder. When the two indices are the same, that is n=m, the resultant tube is said to be of the “arm-chair” (or n,n) type, since when that type of tube is cut perpendicular to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. Due to their metallic nature, with extremely high electrical and thermal conductivity, the arm-chair tubes are a preferred form of SWNT.

Metallic nanotubes can exhibit ballistic conduction, conduction by non-scattered charge carriers. With ballistic conduction, the resistance value becomes independent of length, and the so-called quantum resistance (6.5 kΩ) is observed.

Arc discharge, laser ablation, thermal chemical vapor deposition (hereinafter “CVD”) and plasma enhanced CVD are several of the known methods for manufacturing carbon nanotubes. Both SWNT and multi-walled nanotubes can be produced by the arc discharge and laser ablation methods.

Catalysts supported on a variety of suitable supports can be utilized in the CVD methods to produce carbon nanotubes. A complete understanding of the effects of catalyst formulation, for instance, transition metals (Ni, Co, Fe, etc.), support material, catalyst/support interaction, synthesis temperature and hydrocarbon gas on the diameter and chirality of the carbon nanotubes produced by CVD methods is still being developed. See, for example, Harutyunyan et al, Nanoletters, Vol. 2, No. 5, 2002, pp. 525-530 and U.S. Patent Application Publication No. US 2003/0124717 A1.

SUMMARY

The present teachings satisfy the need for a method of producing cylindrical carbon structures from initially produced cylindrical carbon structures so that the subsequently produced cylindrical carbon structures have the same chirality as the initially produced cylindrical carbon structures.

A method of preparing cylindrical carbon structures by providing a catalyst component on a substrate and a carbon component, and contacting the catalyst component and the carbon component to produce a first cylindrical carbon structure is taught by the present disclosure. The method further includes stopping providing the carbon component, cleaning the catalyst component, and then again providing the carbon component to produce more of the cylindrical carbon structure.

The present teachings further provide single-walled carbon nanotubes prepared by a process including providing a catalyst component on a substrate, providing a carbon component and contacting the catalyst component and the carbon component to produce a first single-walled carbon nanotube having a chirality. Then stopping providing the carbon component, cleaning the catalyst components, and again providing the carbon component to produce a continued first single-walled carbon nanotube, such that the continued first single-walled carbon nanotube has the same chirality as the first single-walled carbon nanotube.

Another method disclosed by the present teachings of preparing single-walled carbon nanotubes includes providing a catalyst component on a substrate, providing a carbon component, contacting the catalyst component and the carbon component to produce a first single-walled carbon nanotube having a chirality and stopping providing the carbon component. The catalyst component is then cleaned, and the carbon component is again provided to produce a continued first single-walled carbon nanotube with the same chirality as the first single-walled carbon nanotube. This procedure is repeated until the continued first single-walled carbon nanotube satisfies a desired characteristic, at which time, the single-walled carbon nanotube is removed from the catalyst component.

DETAILED DESCRIPTION

The present teachings provide a method of preparing cylindrical carbon structures, specifically SWNT, by providing a catalyst component on a substrate, providing a carbon component, contacting the catalyst component and the carbon component to produce a first cylindrical carbon structure, and then stopping the provision of the carbon component. At this point in the method, the catalyst component can be cleaned, and after cleaning, the carbon component can be reintroduced to produce additional cylindrical carbon structure.

The chirality of the first cylindrical carbon structure can be determined after the provision of the carbon component is stopped. The preparation can then be continued by repeating the steps of providing carbon component, contacting the catalyst and carbon components to produce a continued cylindrical carbon structure, stopping the provision of the carbon component, and cleaning the catalyst component, until the cylindrical carbon structure satisfies a desired characteristic.

While it is not presently feasible to produce a cylindrical carbon structure, or SWNT, with a predetermined chirality, in the present disclosure, the chirality of the continued cylindrical carbon structure produced has the same chirality as the first cylindrical carbon structure. The presently disclosed process provides that where given an initial cylindrical carbon structure, preferably an SWNT, with a certain chirality, that cylindrical carbon structure can be, for instance, increased in length with the additional cylindrical carbon structure having the same chirality as the initial cylindrical carbon structure.

The desired characteristic can include, for example, at least one member selected from the group consisting of length, electrical conductivity, thermal conductivity, metallic character, semi-conductor character and non-metallic character. Upon satisfying the desired characteristic the cylindrical carbon structure can be removed from the catalyst component. Alternatively, the production process can be ceased when the efficiency of the process decreases due to build-up of a coating on the catalyst component as described in more detail herein.

The catalyst component can include nanoparticles containing at least one member selected from the group consisting of transition metals, such as, for example, iron, nickel, cobalt, molybdenum, ruthenium and combinations thereof. Of particular interest are catalyst formulations of transition metals and combinations thereof which exhibit resistance to or decreased formation of coatings on the catalyst itself. Typically, the coatings are composed of amorphous carbon, multilayer carbon and metal carbides.

The present method of producing cylindrical carbon structures can utilize either a plasma enhanced CVD method or a thermal CVD method to produce the carbon component as a carbon vapor produced from a carbon source, such as, for example, methane, ethylene, acetylene or carbon dioxide. In the present method, the catalyst component can be heated to a temperature ranging from about 60° C. to about 100° C.

In the CVD methods that can be utilized according to the present disclosure, the catalyst nanoparticle utilized in the method can, after exposure for a period of time to a carbon source, develop a coating or layer of non-reactive material. Various cleaning processes are presented in the present disclosure which clean the catalyst component by reducing any coating present on the catalyst component.

Cleaning the catalyst component refers to using a cleaning method sufficiently active to remove or deactivate, to the extent that cleaning allows subsequent continued production of the cylindrical carbon structure, any coating or build-up present on the catalyst component. Preferably, cleaning the catalyst component includes a cleaning method that does not react, or does not react substantially, with the cylindrical carbon structure.

Oxidation, reduction, dissolution, radiative heating, chemical treatment, plasma treatment and combinations thereof are examples of suitable cleaning methods for removal of the coating on the catalyst component. Examples of chemical treatment include contacting the coating with, for example, water, peroxides and acids. Radiative heating includes exposing the catalyst component and coating to radiation of a wavelength capable of heating primarily the coating and/or the catalyst component to thereby induce oxidation of the coating. Preferably, the radiative heating does not adversely affect either of the catalyst component or the cylindrical carbon structure. Examples of suitable radiation methods include electromagnetic radiation, laser radiation and microwave radiation.

The coating present on the catalyst component typically consists of amorphous carbon, multilayer carbon, metal carbide and combinations thereof. According to present theory, without being limited thereby, as the CVD process continues, non-nanotube forming carbon arrives at the catalyst component and can form, for example, amorphous carbon, multilayer carbon and metal carbide. Each of these formations results in decreased access to the catalyst component for the incoming carbon component and eventually leads to decreased or ceased nanotube growth. According to present theory, these coating components arise in a variety of ways, including incomplete combustion of the supplied hydrocarbon, incomplete formation of cylindrical carbon structures, formation of metal carbides with the metallic elements of the catalyst component, and layering of either or both of incompletely combusted hydrocarbons or incompletely formed cylindrical carbon structures.

The catalyst component can also become less active through the formation of metal oxides on the catalyst. Reduction of the metal oxides back to the metallic state can also improve the catalyst performance, and can in some cases be accomplished during the cleaning of the catalyst component.

The cylindrical carbon structures produced by the present methods can include single-walled carbon nanotubes, double-walled carbon nanotubes and multi-walled carbon nanotubes. Preferably, the present method produces single-walled carbon nanotubes.

The substrate utilized in the presently disclosed methods is not generally restricted, and can include any commonly used substrate. Suitable examples of substrates include, without limitation, silicon substrates, glass substrates, alumina substrates and quartz substrates.

According to the present disclosure, single-walled carbon nanotubes can be prepared by providing a catalyst component on a substrate, providing a carbon component and contacting the catalyst component and the carbon component to produce a first single-walled carbon nanotube having a chirality. After a sufficient amount of the initial SWNT is formed, the carbon component is no longer provided, and the catalyst component can be cleaned. After cleaning, the carbon component can again be provided to produce a continued single-walled carbon nanotube which has the same chirality as the first single-walled carbon nanotube.

This process can further include determining the chirality of the first single-walled carbon nanotube at any point after the provision of the carbon component has ceased. The process can be repeated until single-walled carbon nanotubes satisfying a desired characteristic are produced, or until the catalyst component after cleaning can no produced the continued first single-walled carbon nanotube.

The desired characteristic can be, for instance, length, electrical conductivity, thermal conductivity, metallic character, semi-conductor character and non-metallic character.

The present disclosure further includes a process of preparing single-walled carbon nanotubes by providing a catalyst component on a substrate and a carbon component, then contacting the catalyst component and the carbon component to produce a first single-walled carbon nanotube having a chirality. Stopping the provision of the carbon component can be the next step and allows for the cleaning the catalyst component. Repeating the provision of the carbon component and contacting it with the catalyst component produces a continued first single-walled carbon nanotube with the same chirality as the first single-walled carbon nanotube. This process can be repeated until the continued first single-walled carbon nanotube satisfies a desired characteristic, and then removing the single-walled carbon nanotube from the catalyst component.

The present process of preparing SWNT can utilize either a plasma enhanced CVD method or a thermal CVD method to produce the carbon component as a carbon vapor produced from a carbon source, such as, for example, methane, ethylene, acetylene or carbon dioxide. In the present process, the catalyst component can be heated to a temperature ranging from about 60° C. to about 100° C.

The process can further include determining the chirality of the first produced single-walled carbon nanotube after the provision of the carbon component is ceased.

The desired characteristic exhibited by the continued first SWNT can include, for example, length, electrical conductivity, thermal conductivity, metallic character, semi-conductor character and non-metallic character.

The catalyst component utilized to produce the SWNT can include nanoparticles which contain transition metals, for instance, iron, nickel, cobalt, molybdenum, ruthenium and combinations thereof.

Cleaning the catalyst component can be accomplished by reducing any coating present on the catalyst component. A cleaning method sufficiently active to remove, to the extent that cleaning allows production of the single-walled carbon nanotube, any coating present on the catalyst component is preferable. Furthermore, any cleaning method does not react, or at least does not substantially react, with the single-walled carbon nanotube.

According to the present disclosure, oxidation, reduction, dissolution, radiative heating, chemical treatment, plasma treatment and combinations thereof can all be utilized as cleaning methods. Chemical treatment includes contacting the coating with at least one member selected from the group consisting of water, peroxides and acids. Radiative heating includes exposing the coating to, for example, electromagnetic radiation, laser radiation or microwave radiation.

The chirality of the cylindrical carbon structures or SWNTs can be determined by a variety of methods including Raman characterization, micro Raman characterization, I-V (“current-voltage”) characterization, and STM (“scanning tunneling microscopy”) measurement.

Electromagnetic radiation refers to radiation composed of oscillating electric and magnetic fields and propagated at the speed of light. Examples of electromagnetic radiation include, without limitation, gamma radiation, X-rays, ultraviolet, visible, infrared, microwave and radio waves.

All publications, articles, papers, patents, patent publications, and other references cited herein are hereby incorporated herein in their entireties for all purposes.

The foregoing detailed description of the various embodiments of the present teachings has been provided for the purposes of illustration and description. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the present teachings and their practical application, thereby enabling others skilled in the art to understand the present teachings for various embodiments and with various modifications as are suited to the particular use contemplated. The specific techniques, conditions, materials and reported data set forth in the following examples to illustrate the principles of the present teachings are exemplary and should not be construed as exhaustive or limiting the scope of the present teachings. It is intended that the scope of the present teachings be defined by the following claims and their equivalents.

EXAMPLES Example 1

Ferric nitrate (Fe(NO₃)₃.9H₂O) can be dissolved in 2-propanol at an approximate concentration of 100 μg/mL, and stirred for 15 minutes. A previously prepared silicon dioxide substrate can then be immersed into the iron solution for 15 seconds, rinsed in hexane, and dried in air.

The substrate with the catalyst can then be placed in a tube furnace and reduced under a helium/hydrogen (60/40) gas flow (200 sccm) at 500 C for one hour. The He/H₂ gas mixture can then be replaced with Ar gas, and the temperature increased to 750 C. Once the higher temperature is reached, then methane gas can be added at a flow rate of 20 sccm for 15 minutes, after which time the furnace is cooled to room temperature under a flow of argon. An atomic force microscopy (“AFM”) image can be obtained of the nanotubes.

The resulting supported iron nanoparticles with nanotubes can be cleaned by exposing the sample to a dry air flow (100 sccm) at a temperature of 200 C for thirty minutes.

The tube furnace can then be reheated to 750 C under a flow (200 sccm) of an argon/hydrogen gas mixture. After the nanoparticles reach a steady state temperature, methane can be re-introduced into the tube furnace, at a flow rate of 20 sccm.

After fifteen minutes, the methane flow can be stopped and the apparatus allowed to cool to room temperature under an argon gas flow. The supported iron nanoparticles with nanotubes can then be removed from the tube furnace.

A second AFM image can be obtained. The second AFM image can show that the nanotubes have grown in length while maintaining the same chirality as the initial nanotube.

Example 2

Ferric nitrate (Fe(NO₃)₃.9H₂O) and ammonium molybdate ((NH₄)₆Mo₇O₂₄.4H₂O) at a 1:0.17 Fe:Mo molar ratio can be dissolved in methanol, and then mixed with a methanol suspension of alumina. The suspension can be deposited, drop wise, onto a previously prepared silicon dioxide substrate, and then dried in air.

The substrate with the bimetallic catalyst can then be placed in a tube furnace and reduced under a helium/hydrogen (60/40) gas flow (200 sccm) at 500 C for one hour. The He/H₂ gas mixture can then be replaced with Ar gas, and the temperature increased to 750 C. Once the higher temperature is reached, then methane gas can be added at a flow rate of 20 sccm for 15 minutes, after which time the furnace is cooled to room temperature under a flow of argon. An atomic force microscopy (“AFM”) image can be obtained of the nanotubes.

The supported iron/molybdenum nanoparticles with nanotubes can be cleaned by exposing the sample to a dry air flow (100 sccm) at a temperature of 200 C for thirty minutes.

The tube furnace can then be reheated to 750 C under a flow (200 sccm) of an argon/hydrogen gas mixture. After the nanoparticles reach a steady state temperature, methane can be re-introduced into the tube furnace, at a flow rate of 20 sccm.

After fifteen minutes, the methane flow can be stopped and the apparatus allowed to cool to room temperature under an argon gas flow. The supported Fe/Mo nanoparticles with nanotubes can then be removed from the tube furnace.

A second AFM image can be obtained. The second AFM image can show that the nanotubes have grown in length while maintaining the same chirality as the initial nanotube.

Example 3

Ferric nitrate (Fe(NO₃)₃.9H₂O) can be dissolved in methanol at an approximate concentration of 150 μg/mL, and then mixed with a methanol suspension of alumina. The alumina can have a BET surface area of 150 m²/g. The iron and alumina suspension can be deposited, drop wise, onto a previously prepared silicon dioxide substrate, and then dried in air.

The substrate with the catalyst can then be placed in a tube furnace and reduced under a helium/hydrogen (60/40) gas flow (200 sccm) at 500 C for one hour. The He/H₂ gas mixture can then be replaced with Ar gas, and the temperature increased to 750 C. Once the higher temperature is reached, then methane gas can be added at a flow rate of 20 sccm for 15 minutes, after which time the furnace is cooled to room temperature under a flow of argon. An atomic force microscopy (“AFM”) image can be obtained of the nanotubes.

The supported iron nanoparticles with nanotubes can be cleaned by exposing the sample to a dry air flow (100 sccm) at a temperature of 200 C for thirty minutes.

The tube furnace can then be reheated to 750 C under a flow (200 sccm) of an argon/hydrogen gas mixture. After the nanoparticles reach a steady state temperature, methane can be re-introduced into the tube furnace, at a flow rate of 20 sccm.

After fifteen minutes, the methane flow can be stopped and the apparatus allowed to cool to room temperature under an argon gas flow. The supported iron nanoparticles with nanotubes can then be removed from the tube furnace.

A second AFM image can be obtained. The second AFM image can show that the nanotubes have grown in length while maintaining the same chirality as the initial nanotube. 

1. A method of preparing cylindrical carbon structures comprising: a) providing a catalyst component on a substrate; b) providing a carbon component; c) contacting the catalyst component and the carbon component to produce a first cylindrical carbon structure; d) stopping providing the carbon component; e) cleaning the catalyst component; f) repeating steps b) through e), and g) producing continued first cylindrical carbon structure having the same chirality as the first cylindrical carbon structure.
 2. The method according to claim 1, further comprising: determining the chirality of the first cylindrical carbon structure after step d).
 3. The method according to claim 1, further comprising: repeating step f) until the cylindrical carbon structure satisfies a desired characteristic.
 4. The method according to claim 3, wherein the desired characteristic comprises at least one member selected from the group consisting of length, electrical conductivity, thermal conductivity, metallic character, semi-conductor character and non-metallic character.
 5. The method according to claim 3, further comprising: removing the cylindrical carbon structure from the catalyst component.
 6. The method according to claim 1, wherein the catalyst component comprises nanoparticles containing at least one member selected from the group consisting of iron, nickel, cobalt, molybdenum, ruthenium and combinations thereof.
 7. The method according to claim 1, wherein the carbon component comprises carbon vapor produced by either a plasma enhanced chemical vapor deposition method or a thermal chemical vapor deposition method.
 8. The method according to claim 7, wherein the carbon vapor is produced from a carbon source comprising at least one element selected from the group consisting of methane, ethylene, acetylene and carbon dioxide.
 9. The method according to claim 1, wherein cleaning the catalyst component comprises heating the catalyst component to about 750° C. under a reductive atmosphere.
 10. The method according to claim 9, wherein cleaning the catalyst component comprises utilizing a method sufficiently active to remove, to the extent that cleaning allows production of the continued cylindrical carbon structure, any coating present on the catalyst component.
 11. The method according to claim 9, wherein cleaning the catalyst component comprises utilizing a cleaning method that does not react with the cylindrical carbon structure. 12-13. (canceled)
 14. The method according to claim 9, wherein heating comprises exposing the coating to at least one member selected from the group consisting of electromagnetic radiation, laser radiation and microwave radiation.
 15. The method according to claim 10, wherein the coating comprises at least one member selected from the group consisting of amorphous carbon, multilayer carbon, metal carbide and combinations thereof.
 16. The method according to claim 1, wherein the cylindrical carbon structure comprises at least one member selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes and multi-walled carbon nanotubes.
 17. The method according to claim 1, wherein the cylindrical carbon structure comprises single-walled carbon nanotubes.
 18. The method according to claim 1, wherein the catalyst component is heated to a temperature ranging from about 600° to about 1000° C. during the contacting step.
 19. (canceled)
 20. A method of preparing single-walled carbon nanotubes comprising: a) providing a catalyst component on a substrate; b) providing a carbon component; c) contacting the catalyst component and the carbon component to produce a first single-walled carbon nanotube having a chirality; d) stopping providing the carbon component; e) cleaning the catalyst component; f) repeating steps b) through e) to produce a continued first single-walled carbon nanotube with the same chirality as the first single-walled carbon nanotube; g) repeating step f) until the continued first single-walled carbon nanotube satisfies a desired characteristic; and h) removing the single-walled carbon nanotube from the catalyst component, wherein cleaning the catalyst component comprises heating the catalyst component in a reductive atmosphere.
 21. The method according to claim 20, further comprising: determining the chirality of the first produced single-walled carbon nanotube after step d).
 22. The method according to claim 20, wherein the desired characteristic comprises at least one member selected from the group consisting of length, electrical conductivity, thermal conductivity, metallic character, semi-conductor character and non-metallic character.
 23. The method according to claim 20, wherein the catalyst component comprises nanoparticles containing at least one member selected from the group consisting of iron, nickel, cobalt, molybdenum, ruthenium and combinations thereof.
 24. The method according to claim 20, wherein the carbon component comprises carbon vapor produced by either a plasma enhanced chemical vapor deposition method or a thermal chemical vapor deposition method.
 25. The method according to claim 24, wherein the carbon vapor is produced from a carbon source comprising at least one element selected from the group consisting of methane, ethylene, acetylene and carbon dioxide.
 26. The method according to claim 20, wherein cleaning the catalyst component comprises heating the catalyst component to about 750° C.
 27. The method according to claim 26, wherein cleaning the catalyst component comprises utilizing a method sufficiently active to remove, to the extent that cleaning allows production of the continued single-walled carbon nanotube, any coating present on the catalyst component.
 28. The method according to claim 26, wherein cleaning the catalyst component comprises utilizing a cleaning method that does not react with the single-walled carbon nanotube. 29-30. (canceled)
 31. The method according to claim 26, wherein heating comprises exposing the coating to at least one member selected from the group consisting of electromagnetic radiation, laser radiation and microwave radiation under a reductive atmosphere.
 32. The method according to claim 27, wherein the coating comprises at least one member selected from the group consisting of amorphous carbon, multilayer carbon and metal carbide.
 33. The method according to claim 20, wherein the catalyst component is heated to a temperature ranging from about 600° to about 1000° C. during the contacting step. 